Stabilization of dendritic arbor structure in vivo by CaMKII

Wnt11-R signaling regulates a calcium sensitive EMT event essential for dorsal fin development of Xenopus

In the frog embryo, a sub-population of trunk neural crest (NC) cells undergoes a dorsal route of migration to contribute to the mesenchyme in the core of the dorsal fin. Here we show that a second population of cells, originally located in the dorsomedial region of the somite, also contributes to the fin mesenchyme. We find that the frog orthologue of Wnt11 (Wnt11-R) is expressed in both the NC and somite cell populations that migrate into the fin matrix. Wnt11-R is expressed prior to migration and persists in the mesenchymal cells after they have distributed throughout the fin. Loss of function studies demonstrate that Wnt11-R activity is required for an epithelial to mesenchymal transformation (EMT) event that precedes migration of cells into the fin matrix. In Wnt11-R depleted embryos, the absence of fin core cells leads to defective dorsal fin development and to collapse of the fin structure. Experiments using small molecule inhibitors indicate that dorsal migration of fin core cells depends on calcium signaling through calcium/calmodulin-dependent kinase II (CaMKII). In Wnt11-R depleted embryos, normal migration of NC cells and dorsal somite cells into the fin and normal fin development can be rescued by stimulation of calcium release. These studies are consistent with a model in which Wnt11-R signaling, via a downstream calcium pathway, regulates fin cell migration and, more generally, indicates a role for non-canonical Wnt signaling in regulation of EMT.

Spatiotemporal expression of four isoforms of Ca2+/calmodulin-dependent protein kinase I in brain and its possible roles in hippocampal dendritic growth

Among multifunctional Ca(2+)/calmodulin-dependent protein kinases (CaMKs), CaMKI has been shown to comprise a family of four structurally related isoforms (alpha, beta, gamma, and delta) encoded by separate genes with abundant expression in mature brain. In this study, we first examined the developmental gene expression of the four isoforms of CaMKI in mouse brain with special attention to the hippocampal formation by in situ hybridization analysis. The four isoforms of CaMKI were found to exhibit distinct spatiotemporal expression during neuronal development. We also examined the functional involvement of CaMKI in the dendritic formation of cultured hippocampal neurons. The overexpression of kinase-dead mutants of CaMKI reduced the average dendritic length of the transfected neurons without any significant effects on the number of primary dendrites and the branching index. Our present findings provide the detailed anatomical information on the developmental expression of the four isoforms of CaMKI in mouse brain, which represents the possible functional involvement of CaMKI in the basal dendritic growth of hippocampal neurons.

Aberrant dendritic branching and sensory inputs in the superficial dorsal horn of mice lacking CaMKIIα autophosphorylation

Superficial dorsal horn neurones undergo marked structural and functional activity-dependent development during the early postnatal period, but little is known about the molecular mechanisms underlying these changes. Calcium signalling, through activation and autophosphorylation of CaMKII, has been shown to play a major role in the maturation of neuronal morphology and connectivity in the cortex. Here, we show that the normal structural and functional development of superficial dorsal horn neurones requires CaMKII autophosphorylation at the Thr286 residue. The dendritic branching of neurones from mice containing a point mutation at this site (T286A) was significantly increased compared with wild-type littermates. This was accompanied by significant increases in receptive field size, recorded from intact preparations. Whole-cell patch clamp recordings of superficial dorsal horn slices revealed a selective deficit in low-threshold A fibre-evoked synaptic input. These results show that CaMKII autophosphorylation is required for the normal development of spinal sensory circuits.

Vasoactive Intestinal Peptide and PACAP38 Control N-Methyl-D-aspartic Acid-induced Dendrite Motility by Modifying the Activities of Rho GTPases and Phosphatidylinositol 3-Kinases

Dendrite morphogenesis is highly dynamic and characterized by the addition and elongation of processes and also by their selective maintenance, retraction, and elimination. Glutamate can influence these events via N-methyl-d-aspartic acid (NMDA) receptors. The neuropeptides vasoactive intestinal peptide and pituitary adenylyl cyclase-activating polypeptide-38 (PACAP38) affect neurogenesis and differentiation in the developing nervous system. We report here that the peptides and NMDA acted synergistically on dendrite and branch formation. In stage III hippocampal neurons, NMDA increased not only the addition but also the elimination of new dendrites and branches by activating Rac and Cdc42 and phosphatidylinositol 3-kinases, respectively. When applied alone, the neuropeptides did not influence dendrite or branch formation. However, they reduced the elimination of newly formed dendrites and branches caused by NMDA by preventing the NMDA-induced activation of phosphatidylinositol 3-kinases. This led to the formation of persistent dendrites and branches. Additional timelapse studies on the dynamics of dendrite elongation showed alternating periods of elongation and retraction. Phosphatidylinositol 3-kinases increased the velocities of dendrite elongation and retraction, whereas the neuropeptides prolonged the periods of elongation. By modifying NMDA-induced activation of Rho GTPases and phosphatidylinositol 3-kinases, vasoactive intestinal peptide and PACAP38 could play an important role in the control of dendrite growth and branching during development and in response to neuronal activity.

AMPA receptors regulate experience-dependent dendritic arbor growth in vivo

The size and shape of neuronal dendritic arbors affect the number and type of synaptic inputs, as well as the complexity and function of brain circuits. In the intact brain, dendritic arbor growth and the development of excitatory glutamatergic synapse are concurrent. Consequently, it has been difficult to resolve whether synaptic inputs drive dendritic arbor development. Here, we test the role of AMPA receptor (AMPAR)-mediated glutamatergic transmission in dendrite growth by expressing peptides corresponding to the intracellular C-terminal domains of AMPAR subunits GluR1 (GluR1Ct) and GluR2 (GluR2Ct) in optic tectal neurons of the Xenopus retinotectal system. These peptides significantly reduce AMPAR synaptic transmission in transfected neurons while leaving visual system circuitry intact. Daily in vivo imaging over 5 days revealed that GluR1Ct or GluR2Ct expression dramatically impaired dendrite growth, resulting in less complex arbors than controls. Time-lapse images collected at 2-h intervals over 6 h show that both GluR1Ct and GluR2Ct decrease branch lifetimes. Ultrastructural analysis indicates that synapses formed onto neurons expressing the GluRCt are less mature than synapses onto control neurons. These data suggest that the failure to form complex arbors is due to reduced stabilization of new synapses and dendritic branches. Although visual stimulation increases dendritic arbor growth rates in control tectal neurons, a weak postsynaptic response to visual experience in GluRCt-expressing cells leads to retraction of branches. These results indicate that AMPAR-mediated transmission underlies experience-dependent dendritic arbor growth by stabilizing branches, and support a competition-based model for dendrite growth.

Developmental distribution of CaM kinase II in the antennal lobe of the sphinx moth Manduca sexta

The antennal lobe (primary olfactory center of insects) is completely reorganized during metamorphosis. This reorganization is accompanied by changing patterns of calcium signaling in neurons and glial cells. In the present study, we investigated the developmental distribution of a major calcium-dependent protein, viz., calcium/calmodulin-dependent protein kinase II (CaM kinase II), in the antennal lobe of the sphinx moth Manduca sexta by using a monoclonal antibody. During synaptogenesis (developmental stages 6-10), we found a redistribution of CaM kinase II immunoreactivity, from a homogeneous distribution in the immature neuropil to an accumulation in the neuropil of the glomeruli. CaM kinase II immunoreactivity was less intense in olfactory receptor axons of the antennal nerve and antennal lobe glial cells. Western blot analysis revealed a growing content of CaM kinase II in antennal lobe tissue throughout metamorphosis. Injection of the CaM kinase inhibitor KN-93 into pupae resulted in a reduced number of antennal lobe glial cells migrating into the neuropil to form borders around glomeruli. The results suggest that CaM kinase II is involved in glial cell migration.

Activity-dependent dendritic arborization mediated by CaM-kinase I activation and enhanced CREB-dependent transcription of Wnt-2

Members of the Wnt signaling family are important mediators of numerous developmental events, including activity-dependent dendrite development, but the pathways regulating expression and secretion of Wnt in response to neuronal activity are poorly defined. Here, we identify an NMDA receptor-mediated, Ca2+-dependent signaling pathway that couples neuronal activity to dendritic arborization through enhanced Wnt synthesis and secretion. Activity-dependent dendritic outgrowth and branching in cultured hippocampal neurons and slices is mediated through activation by CaM-dependent protein kinase kinase (CaMKK) of the membrane-associated gamma isoform of CaMKI. Downstream effectors of CaMKI include the MAP-kinase pathway of Ras/MEK/ERK and the transcription factor CREB. A serial analysis of chromatin occupancy screen identified Wnt-2 as an activity-dependent CREB-responsive gene. Neuronal activity enhances CREB-dependent transcription of Wnt-2, and expression of Wnt-2 stimulates dendritic arborization. This novel signaling pathway contributes to dynamic remodeling of the dendritic architecture in response to neuronal activity during development.

Bi-directional control of motor neuron dendrite remodeling by the calcium permeability of AMPA receptors

Motor neurons express particularly high levels of the AMPA receptor subunit GluR1(Q)flip (GluR1(Q)i) during the period in early postnatal life when their dendritic tree grows and becomes more branched. To investigate how GluR1-containing AMPA receptors contribute to dendrite morphogenesis, we characterized a mutant form of GluR1 (containing a histidine in the Q/R editing site) with unique electrophysiological properties. Most notably, AMPA receptors assembled from GluR1(H)i display less calcium permeability than AMPA receptors assembled from GluR1(Q)i. Expression of GluR1(Q)i in vivo or in vitro led to an increase in dendrite branching with no net change in the overall tree size while GluR1(H)i led to a loss of branches and a net reduction in overall tree size. GluR1(H)i-dependent dendrite atrophy is mediated by protein phosphatase 2B. The results suggest that the electrophysiological properties of cell surface AMPA receptors, specifically their permeability to calcium, can be a central determinant of whether the dendrites undergo activity-dependent branching or atrophy.

P2Y1and P2X7receptors induce calcium/calmodulin-dependent protein kinase II phosphorylation in cerebellar granule neurons

The activation of nucleotide receptors-- both ionotropic, P2X, and most of metabotropic, P2Y-- increases intracellular calcium concentration, resulting in calcium/calmodulin-dependent protein kinase II (CaMKII) activation. Stimulation of cerebellar granule neurons in culture-- with different P2X and P2Y agonists and their effect on CaMKII phosphorylation-- was studied using immunocytochemical and microfluorimetrical techniques. P2X agonist: 2'-3'-o-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP), alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP) and diadenosine pentaphosphate (Ap(5)A); and P2Y agonists: 2-(methylthyo)-adenosine diphosphate (2MeSADP) and uridine 5'-bisphosphate (UDP); tested induced a CaMKII phosphorylation but with a different immunostaining pattern in each group. Stimulation with 2MeSADP induced a Ca(2+) release from intracellular stores and a significant CaMKII phosphorylation in cell somas and neurites. This agrees with the subcellular distribution of P2Y(1). MRS 2179, a specific P2Y(1) inhibitor, antagonized the 2MeSADP effect. On the other hand, cerebellar granule neuron stimulation with BzATP, in Mg(2+)-free conditions, produced extracellular calcium entrance and, as a result, a significant increase in CaMKII phosphorylation mostly in fibres, which correspond with P2X(7) subdistribution. Immunocytochemical and microfluorimetrical experiments, using Zn(2+) and Brilliant Blue G (BBG), as a specific P2X(7) antagonist, confirmed that BzATP was acting through the P2X(7) receptor. These results indicate that P2Y(1) and P2X(7) produce a significant increase in CaMKII phosphorylation, but show important differences in subcellular distribution and in effect duration. P2X(7) activation in granule neurons is not associated with pore formation, according to the absence of YO-PRO-1 fluorescence. The abundant presence of P2X(7) at the synaptic structures suggests a relevant role played by this receptor in synaptic plasticity.

BDNF increases synapse density in dendrites of developing tectal neurons in vivo

Neuronal connections are established through a series of developmental events that involve close communication between pre- and postsynaptic neurons. In the visual system, BDNF modulates the development of neuronal connectivity by influencing presynaptic retinal ganglion cell (RGC) axons. Increasing BDNF levels in the optic tectum of Xenopus tadpoles significantly increases both axon arborization and synapse density per axon terminal within a few hours of treatment. Here, we have further explored the mechanisms by which BDNF shapes synaptic connectivity by imaging tectal neurons, the postsynaptic partners of RGCs. Individual neurons were co-labeled with DsRed2 and a GFP-tagged postsynaptic density protein (PSD95-GFP) to visualize dendritic morphology and postsynaptic specializations simultaneously in vivo. Immunoelectron microscopy confirmed that PSD95-GFP predominantly localized to ultrastructurally identified synapses. Time-lapse confocal microscopy of individual, double-labeled neurons revealed a coincident, activity-dependent mechanism of synaptogenesis and axon and dendritic arbor growth, which is differentially modulated by BDNF. Microinjection of BDNF into the optic tectum significantly increased synapse number in tectal neuron dendritic arbors within 24 hours, without significantly influencing arbor morphology. BDNF function-blocking antibodies had opposite effects. The BDNF-elicited increase in synapse number complements the previously observed increase in presynaptic sites on RGC axons. These results, together with the timescale of the response by tectal neurons, suggest that the effects of BDNF on dendritic synaptic connectivity are secondary to its effects on presynaptic RGCs. Thus, BDNF influences synaptic connectivity in multiple ways: it enhances axon arbor complexity expanding the synaptic territory of the axon, while simultaneously coordinating synapse formation and stabilization with individual postsynaptic cells.

Developmental changes of CaMKII localization, activity and function during postembryonic CNS remodelling in Manduca sexta

Insect metamorphosis is a compelling example of postembryonic remodelling of neuronal structure and synaptic connectivity as larval and adult behaviours place distinct demands on the CNS. Holometabolous insects such as the moth Manduca sexta have long served as suitable models for the study of steroid effects on CNS remodelling, but activity and calcium-dependent mechanisms have been found to act in concert with hormonal signals. This study examines developmental changes in the localization and the activational state of CaMKII during postembryonic Manduca CNS remodelling. Western blotting, CaMKII purification and autophosphorylation with gamma(32)P-ATP indicate that the lepidopteran CNS may contain only one CaMKII isoform. In situ immunohistochemistry reveals developmental changes in the expression patterns of CaMKII in different types of thoracic neurons and in different neuronal compartments. Early pupal life is characterized by an increase in postsynaptic CaMKII localization, which coincides with a developmental increase in CaMKII activation. Both events correlate temporally with motoneuron dendritic filopodia collapse and rapid synaptogenesis, indicating a possible functional role for CaMKII for the postembryonic development of invertebrate motor circuitry. Substrate phosphorylation assays demonstrate that CaMKII activity in the ventral nerve cord reflects changes in calcium influx through voltage-activated channels as occurring in vivo during normal development.

BDNF mRNA expression and protein localization are changed in age-related hearing loss

A decline in neuronal plasticity during the adult life span has been proposed to be associated with a reduced level of the effectors of plasticity responses (e.g., BDNF). Alteration of plasticity is also correlated with age-related hearing loss (presbycusis), but to date no detailed studies of BDNF expression have been performed in the young or aging mature cochlea. We have used rat and gerbil animal models for presbycusis, which displayed hearing loss in the final third of the animals' natural life span. We demonstrate for the first time a co-localization of BDNF protein, transcripts III and IV in cochlear neurons with a declining distribution towards low-frequency processing cochlear turns. BDNF protein was also found within the neuronal projections of the cochlea. A significant reduction of BDNF transcripts in high-frequency processing cochlear neurons was observed during aging, though this did not coincide with a major reduction of BDNF protein. In contrast, BDNF protein in peripheral and central projections was drastically reduced. Our results suggest that reduced BDNF protein levels in auditory nerves over age may be a crucial factor in the altered brainstem plasticity observed during presbycusis.

NMDA receptor-dependent regulation of axonal and dendritic branching

In the rodent trigeminal principal nucleus (PrV), trigeminal afferent terminals and postsynaptic cells form discrete modules ("barrelettes") that replicate the patterned array of whiskers and sinus hairs on the snout. Barrelette neurons of the PrV relay whisker-specific patterns to the contralateral thalamus and, subsequently, to the primary somatosensory barrel cortex. Genetic impairment of NMDA receptor (NMDAR) function blocks development of barrelettes in the PrV. Underlying cellular and functional defects are not known. Here, we examined morphological differentiation of whisker afferents, dendritic differentiation of barrelette cells, and their electrophysiological properties in mice with genetic perturbations of the essential subunit NR1 of NMDARs. We show that in NR1 gene knock-down (KD) and knock-out mice, whisker afferents begin their embryonic development normally but, over time, fail to segregate into patches, and instead they develop exuberant terminal arbors spanning most of the PrV. Postnatal NR1KD barrelette cells, with significantly reduced NMDA currents, retain their membrane and synaptic properties but develop longer dendrites with no orientation preference. These results indicate that NMDARs regulate growth of presynaptic terminal arbors and postsynaptic dendritic branching, thereby leading to consolidation of synapses and patterning of presynaptic and postsynaptic elements.

Control of dendritic arborization by the phosphoinositide-3'-kinase-Akt-mammalian target of rapamycin pathway

The molecular mechanisms that determine the size and complexity of the neuronal dendritic tree are unclear. Here, we show that the phosphoinositide-3' kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) signaling pathway promotes the growth and branching of dendrites in cultured hippocampal neurons. Constitutively active mutants of Ras, PI3K, and Akt, or RNA interference (RNAi) knockdown of lipid phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome Ten), induced growth and elaboration of dendrites that was blocked by mTOR inhibitor rapamycin and/or by overexpression of eIF-4E binding protein 1 (4E-BP1), which inhibits translation of 5' capped mRNAs. The effect of PI3K on dendrites was lost in more mature neurons (>14 d in vitro). Dendritic complexity was reduced by inhibition of PI3K and by RNAi knockdown of mTOR or p70 ribosomal S6 kinase (p70S6K, an effector of mTOR). A rapamycin-resistant mutant of mTOR "rescued" the morphogenetic effects of PI3K in the presence of rapamycin. By regulating global and/or local protein translation, and as a convergence point for multiple signaling pathways, mTOR could play a central role in the control of dendrite growth and branching during development and in response to activity.

The development of neuronal morphology in insects

Neurons are highly polarized cells with some regions specified for information input--typically the dendrites--and others specialized for information output--the axons. By extending to a specific location and branching in a specific manner, the processes of neurons determine at a fundamental level how the nervous system is wired to produce behavior. Recent studies suggest that relatively small changes in neuronal morphology could conceivably contribute to striking behavioral distinctions between invertebrate species. We review recent data that begin to shed light on how neurons extend dendrites to their targets and acquire their particular branching morphologies, drawing primarily on data from genetic model organisms. We speculate about how and why the actions of these genes might facilitate the diversification of dendritic morphology.

Regulation of dendritic morphogenesis by Ras-PI3K-Akt-mTOR and Ras-MAPK signaling pathways

Dendritic arborization and spine formation are critical for the functioning of neurons. Although many proteins have been identified recently as regulators of dendritic morphogenesis, the intracellular signaling pathways that control these processes are not well understood. Here we report that the Ras-phosphatidylinositol 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) signaling pathway plays pivotal roles in the regulation of many aspects of dendrite formation. Whereas the PI3K-Akt-mTOR pathway alone controlled soma and dendrite size, a coordinated activation together with the Ras-mitogen-activated protein kinase signaling pathway was required for increasing dendritic complexity. Chronic inhibition of PI3K or mTOR reduced soma and dendrite size and dendritic complexity, as well as density of dendritic filopodia and spines, whereas a short-term inhibition promoted the formation of mushroom-shaped spines on cells expressing constitutively active mutants of Ras, PI3K, or Akt, or treated with the upstream activator BDNF. Together, our data underscore the central role of a spatiotemporally regulated key cell survival and growth pathway on trophic regulation of the coordinated development of dendrite size and shape.

Calcium/calmodulin-dependent protein kinase II alters structural plasticity and cytoskeletal dynamics in Drosophila

Drosophila dendritic arborization (da) neurons contain subclasses of neurons with distinct dendritic morphologies. We investigated calcium/calmodulin-dependent protein kinase II (CaMKII) regulation of dendritic structure and dynamics in vivo using optically transparent Drosophila larvae. CaMKII increases the dynamic nature and formation of dendritic filopodia throughout larval development but only affects neurons that normally contain dendritic filopodia. In parallel, we examined the effects of Rac1 activity on dendritic structure to explore signaling specificity. In contrast to CaMKII activity, Rac1 does not alter filopodia stability but instead causes de novo filopodia formation on all da neurons. Although both mediators increase cytoskeletal turnover, measured by fluorescence recovery after photobleaching experiments, only CaMKII increases the dynamic nature of dendritic filopodia. CaMKII signaling thus appears to use mechanisms and machinery distinct from Rac1 signaling. This study illustrates a molecular means of uncoupling cytoskeletal regulation from morphological regulation. Our results suggest that Drosophila dendritic filopodia may share some cytoskeletal regulatory mechanisms with mammalian dendritic filopodia. Furthermore, general dendrite cytoskeletal compartmentalization is conserved in multipolar neurons.

Netrin-1 induces axon branching in developing cortical neurons by frequency-dependent calcium signaling pathways

A single axon can innervate multiple targets by collateral branching. Axon branching is thus essential for establishing CNS connectivity. However, surprisingly little is known about the mechanisms by which branching is regulated. Axons often stop elongating before branches develop and anatomical and molecular data suggest that axon branching occurs independent of axon outgrowth. We found that netrin-1 dramatically increases cortical axon branching. Here, we sought to identify intracellular signaling components involved in netrin-1-induced axon branching. Using live cell imaging of dissociated developing cortical neurons, we show that netrin-1 rapidly increases the frequency of repetitive calcium transients. These transients are often restricted to small regions of the axon. Simultaneous imaging of calcium activity and development of axon branches revealed that Ca2+ transients coincide spatially and temporally with protrusion of branches from the axon. Remarkably, fully formed branches with motile growth cones could develop de novo within 20 min. Netrin-1-induced Ca2+ transients involve release from intracellular stores and Ca2+ signaling is essential for netrin-1-induced axon branching. Using techniques to overexpress or suppress kinase activity, we find that calcium/calmodulin-dependent protein kinase II (CaMKII) and mitogen-activated protein kinase (MAPK) are major downstream targets of the netrin-1 calcium signaling pathway and are required for axon branching. CaMKII, but not MAPK, is also involved in axon outgrowth. The role of CaMKII and MAPKs in axon branching is consistent with the sensitivity of these kinases to changes in the frequency Ca2+ transients. Together, these novel findings define calcium signaling mechanisms required for development of new axon branches promoted by a guidance cue.

A mechanism for Ca2+/calmodulin-dependent protein kinase II clustering at synaptic and nonsynaptic sites based on self-association

The activity of Ca2+/calmodulin-dependent protein kinase II (CaMKII) plays an integral role in regulating synaptic development and plasticity. We designed a live-cell-imaging approach to monitor an activity-dependent clustering of green fluorescent protein (GFP)-CaMKII holoenzymes, termed self-association, a process that we hypothesize contributes to the translocation of CaMKII to synaptic and nonsynaptic sites in activated neurons. We show that GFP-CaMKII self-association in human embryonic kidney 293 (HEK293) cells requires a catalytic domain and multimeric structure, requires Ca2+ stimulation and a functional Ca2+/CaM-binding domain, is regulated by cellular pH and Thr286 autophosphorylation, and has variable rates of dissociation depending on Ca2+ levels. Furthermore, we show that the same rules that govern CaMKII self-association in HEK293 cells apply for extrasynaptic and postsynaptic translocation of GFP-CaMKII in hippocampal neurons. Our data support a novel mechanism for targeting CaMKII to postsynaptic sites after neuronal activation. As such, CaMKII may form a scaffold that, in combination with other synaptic proteins, recruits and localizes additional proteins to the postsynaptic density. We discuss the potential function of CaMKII self-association as a tag of synaptic activity.

Integrin-mediated dendrite branch maintenance requires Abelson (Abl) family kinases

Dendrite arbor structure is a critical determinant of nervous system function that must be actively maintained throughout life, but the signaling pathways that regulate dendrite maintenance are essentially unknown. We report that the Abelson (Abl) and Abl-related gene (Arg) nonreceptor tyrosine kinases are required for maintenance of cortical dendrites in the mouse brain. arg-/- cortical dendrites initially develop normally and are indistinguishable from wild-type dendrites at postnatal day 21. Dendrite branches are not efficiently maintained in arg-/- neurons, leading to a reduction in dendrite arbor size by early adulthood. More severe dendrite loss is observed in abl-/-arg-/- neurons. Elevation of Arg kinase activity in primary cortical neurons promotes axon and dendrite branching. Activation of integrin receptors by adhesion to laminin-1 or Semaphorin 7A also promotes neurite branching in cortical neurons, but this response is absent in arg-/- neurons because of the reduced dynamic behavior of mutant neurite branches. These data suggest that integrin signaling through Abl and Arg support cortical dendrite branch maintenance by promoting dendrite branch dynamics in response to adhesive cues.

Molecular determinants of the face map development in the trigeminal brainstem

The perception of external sensory information by the brain requires highly ordered synaptic connectivity between peripheral sensory neurons and their targets in the central nervous system. Since the discovery of the whisker-related barrel patterns in the mouse cortex, the trigeminal system has become a favorite model for study of how its connectivity and somatotopic maps are established during development. The trigeminal brainstem nuclei are the first CNS regions where whisker-specific neural patterns are set up by the trigeminal afferents that innervate the whiskers. In particular, barrelette patterns in the principal sensory nucleus of the trigeminal nerve provide the template for similar patterns in the face representation areas of the thalamus and subsequently in the primary somatosensory cortex. Here, we describe and review studies of neurotrophins, multiple axon guidance molecules, transcription factors, and glutamate receptors during early development of trigeminal connections between the whiskers and the brainstem that lead to emergence of patterned face maps. Studies from our laboratories and others' showed that developing trigeminal ganglion cells and their axons depend on a variety of molecular signals that cooperatively direct them to proper peripheral and central targets and sculpt their synaptic terminal fields into patterns that replicate the organization of the whiskers on the muzzle. Similar mechanisms may also be used by trigeminothalamic and thalamocortical projections in establishing patterned neural modules upstream from the trigeminal brainstem.

A novel forward genetic screen for identifying mutations affecting larval neuronal dendrite development in Drosophila melanogaster

Vertebrate and invertebrate dendrites are information-processing compartments that can be found on both central and peripheral neurons. Elucidating the molecular underpinnings of information processing in the nervous system ultimately requires an understanding of the genetic pathways that regulate dendrite formation and maintenance. Despite the importance of dendrite development, few forward genetic approaches have been used to analyze the latest stages of dendrite development, including the formation of F-actin-rich dendritic filopodia or dendritic spines. We developed a forward genetic screen utilizing transgenic Drosophila second instar larvae expressing an actin, green fluorescent protein (GFP) fusion protein (actin::GFP) in subsets of sensory neurons. Utilizing this fluorescent transgenic reporter, we conducted a forward genetic screen of >4000 mutagenized chromosomes bearing lethal mutations that affected multiple aspects of larval dendrite development. We isolated 13 mutations on the X and second chromosomes composing 11 complementation groups affecting dendrite outgrowth/branching, dendritic filopodia formation, or actin::GFP localization within dendrites in vivo. In a fortuitous observation, we observed that the structure of dendritic arborization (da) neuron dendritic filopodia changes in response to a changing environment.

Dendritic morphogenesis of cerebellar Purkinje cells through extension and retraction revealed by long-term tracking of living cells in vitro

Cerebellar Purkinje cells have the most elaborate dendritic trees among the neurons in the CNS. To investigate the dynamic aspects of dendritic morphogenesis of Purkinje cells, we performed a long-term analysis of living cells in cerebellar cell cultures derived from glutamate decarboxylase 67-green fluorescent protein mice. Most Purkinje cells had several primary dendrites during the 25-day culture period. Repeated observation of green fluorescent protein-expressing Purkinje cells over a period of 10-25 days in vitro demonstrated that not only extension, but also retraction of primary dendrites occurred during this culture period. Interestingly, both extension and retraction of primary dendrites were active between 10 and 15 days in vitro, and retraction of a primary dendrite occurred concomitantly with elongation of other primary dendrites in the same cell. Analysis of the morphological characteristics of the retracted primary dendrites demonstrated that shorter and less branched primary dendrites tended to retract. Furthermore, treatment with an inhibitor of calcium/calmodulin-dependent protein kinase II reduced the number of primary dendrites specifically during 5-15 days in vitro, the culture period when the extension and retraction of primary dendrites occurred actively. Blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate-type glutamate receptors also reduced the number of primary dendrites during the same culture period, while inhibition of glutamate transporters increased the number. These findings suggest that the final morphology of Purkinje cells is achieved not only through extension, but also through retraction of their dendrites, and that calcium/calmodulin-dependent protein kinase II and neuronal activity are involved in this dendritic morphogenesis.

Synaptic activity and F-actin coordinately regulate CaMKIIalpha localization to dendritic postsynaptic sites in developing hippocampal slices

We examined the timing and mechanisms of CaMKIIalpha recruitment to nascent synapses of developing rat hippocampal pyramidal neurons in slice culture. Time-lapse confocal imaging shows that GFP-CaMKIIalpha in transfected neurons accumulates in spines as they are forming, and loss of CaMKIIalpha coincides with spine destabilization. Immunolabeling shows that endogenous CaMKIIalpha is concentrated at postsynaptic sites in spines under ambient slice culture conditions, and this is not disrupted by short-term (3 h) synaptic activity blockade or Latrunculin-induced F-actin depolymerization. However, the combination of activity blockade and F-actin depolymerization significantly reduces synaptic CaMKIIalpha. Conversely, postsynaptic activation induces synaptic recruitment of CaMKIIalpha even in the presence of F-actin depolymerizing drugs. Thus, synaptic-activity-dependent mechanisms and (synaptic activity-independent) F-actin-based mechanisms are individually sufficient and act in parallel to localize CaMKIIalpha to the dendritic spine compartment. Moreover, the timing of CaMKIIalpha recruitment to developing spines suggests a role for CaMKIIalpha in spine assembly and maintenance.

A CaMKII-NeuroD signaling pathway specifies dendritic morphogenesis

The elaboration of dendrites is fundamental to the establishment of neuronal polarity and connectivity, but the mechanisms that underlie dendritic morphogenesis are poorly understood. We found that the genetic knockdown of the transcription factor NeuroD in primary granule neurons including in organotypic cerebellar slices profoundly impaired the generation and maintenance of dendrites while sparing the development of axons. We also found that NeuroD mediated neuronal activity-dependent dendritogenesis. The activity-induced protein kinase CaMKII catalyzed the phosphorylation of NeuroD at distinct sites, including endogenous NeuroD at Ser336 in primary neurons, and thereby stimulated dendritic growth. These findings uncover an essential function for NeuroD in granule neuron dendritic morphogenesis. Our study also defines the CaMKII-NeuroD signaling pathway as a novel mechanism underlying activity-regulated dendritic growth that may play important roles in the developing and mature brain.

Ontogeny of 5-HT1A receptor expression in the developing hippocampus

Serotonin (5-HT) has long been implicated in a number of neurodevelopmental processes including neuronal cell division, migration, neurite outgrowth, and synapse formation. However, relatively little is known about how these effects are mediated during normal brain development in vivo and the identity of the receptor subtypes involved in mediating these effects. In recent years, a number of pharmacological studies have suggested a role for the serotonin 1A (5HT1A) receptor subtype in mediating the developmental effects of 5-HT in the hippocampus. These studies, however, have been difficult to interpret due to lack of information regarding the expression and distribution of 5HT1A in the developing brain and hippocampus in particular. In the current study, specific anti-5-HT1A antibodies, developed in our laboratory [F.C. Zhou, T.D. Patel, D. Swartz, Y. Xu, M.R. Kelley, Production and characterization of an anti-serotonin 1A receptor antibody which detects functional 5-HT1A binding sites, Brain Res Mol Brain Res, 69 (1999) 186-201], were utilized to map the ontogeny and distribution of the 5HT1A receptor protein in the developing rat hippocampus through embryonic and early postnatal life. This is the first such study of 5-HT1A expression in the developing rat brain. Our findings revealed that expression of the 5HT1A receptor emerges during the initial stages of embryonic hippocampal development. Remarkably, most if not all hippocampal neurons begin to express 5HT1A shortly upon completion of their terminal mitosis. We found that 5HT1A is initially concentrated around the cell bodies and later becomes more sparsely distributed along the dendrites after the neurons have matured. In addition to postmitotic neurons, we have observed that S100 and GFAP positive glia transiently express 5HT1A during early postnatal development of the hippocampus. These findings demonstrate that the 5-HT1A receptor is positioned to mediate developmental effects of serotonin in the hippocampus. Furthermore, the temporal patterns of expression suggest a role for 5-HT1A in postmitotic events such as neuronal migration, neurite outgrowth, and phenotypic differentiation.

Neurobiology of mood, anxiety, and emotions as revealed by studies of a unique antidepressant: tianeptine

Recent studies have provided evidence that structural remodeling of certain brain regions is a feature of depressive illness, and the postulated underlying mechanisms contribute to the idea that there is more to antidepressant actions that can be explained exclusively by a monoaminergic hypothesis. This review summarizes recent neurobiological studies on the antidepressant, tianeptine (S-1574, [3-chloro-6-methyl-5,5-dioxo-6,11-dihydro-(c,f)-dibenzo-(1,2-thiazepine)-11-yl) amino]-7 heptanoic acid, sodium salt), a compound with structural similarities to the tricyclic antidepressant agents, the efficacy and good tolerance of which have been clearly established. These studies have revealed that the neurobiological properties of tianeptine involve the dynamic interplay between numerous neurotransmitter systems, as well as a critical role of structural and functional plasticity in the brain regions that permit the full expression of emotional learning. Although the story is far from complete, the schema underlying the effect of tianeptine on central plasticity is the most thoroughly studied of any antidepressants. Effects of tianeptine on neuronal excitability, neuroprotection, anxiety, and memory have also been found. Together with clinical data on the efficacy of tianeptine as an antidepressant, these actions offer insights into how compounds like tianeptine may be useful in the treatment of neurobiological features of depressive disorders.

Regulation of dendritic growth and plasticity by local and global calcium dynamics

The dendritic arbors of neurons are organized into unique spatial patterns that are well suited for their specific functions. Although the intracellular signaling pathways that determine dendritic arbor size and branching patterns during development are not completely understood, it is evident that neurotransmission-mediated elevation in intracellular calcium levels ([Ca2+]i) plays a major role. Recent advances in calcium imaging and genetic approaches have provided new insight into how calcium acts to regulate dendritic growth and remodeling. Global increases in [Ca2+]i that occur upon neuronal depolarization control dendritic arbor growth by regulating transcription. However, the detailed branching patterns of dendritic arbors are regulated by local calcium signaling. Calcium-induced calcium release (CICR) from intracellular stores acts to locally stabilize dendritic branches, presumably triggered by neurotransmission upon contact with newly established inputs. The effects of global and local increases in [Ca2+]i on dendritic structure are cell type- and age-dependent. Unraveling the signaling pathways evoked by local and global rises in [Ca2+]i that shape the developing dendritic arbor at each developmental stage remains challenging but highly important.

Calcium Signaling and the Control of Dendritic Development

Dendrites serve a critical role in neuronal information processing as sites of synaptic integration. The morphological diversity of dendritic architecture reflects specialized strategies that neurons have evolved to detect and process incoming information. Recent observations suggest that calcium signals exert an important influence on neuronal morphology by regulating the growth and branching of dendrites and the formation of dendritic spines. Calcium signals appear to influence branch dynamics by affecting the cytoskeleton near the site of calcium entry, whereas calcium-dependent dendritic growth involves activation of a transcriptional program.

Wnt11-R, a protein closely related to mammalian Wnt11, is required for heart morphogenesis in Xenopus

Wnt11 is a secreted protein that signals through the non-canonical planar cell polarity pathway and is a potent modulator of cell behavior and movement. In human, mouse, and chicken, there is a single Wnt11 gene, but in zebrafish and Xenopus, there are two genes related to Wnt11. The originally characterized Xenopus Wnt11 gene is expressed during early embryonic development and has a critical role in regulation of gastrulation movements. We have identified a second Xenopus Wnt11-Related gene (Wnt11-R) that is expressed after gastrulation. Sequence comparison suggests that Xenopus Wnt11-R, not Wnt11, is the ortholog of mammalian and chicken Wnt11. Xenopus Wnt11-R is expressed in neural tissue, dorsal mesenchyme derived from the dermatome region of the somites, the brachial arches, and the muscle layer of the heart, similar to the expression patterns reported for mouse and chicken Wnt11. Xenopus Wnt11-R exhibits biological properties similar to those previously described for Xenopus Wnt11, in particular the ability to activate Jun-N-terminal kinase (JNK) and to induce myocardial marker expression in ventral marginal zone (VMZ) explants. Morpholino inhibition experiments demonstrate, however, that Wnt11-R is not required for cardiac differentiation, but functions in regulation of cardiac morphogenesis. Embryos with reduced Wnt11-R activity exhibit aberrant cell-cell contacts within the myocardial wall and defects in fusion of the nascent heart tube.

Developmental stage-dependent modulation of synapses by postsynaptic expression of activated calcium/calmodulin-dependent protein kinase II

In Drosophila neuromuscular junctions, there is a unique system which consists of two neighboring muscles (M6 and M7) innervated by the same neurons and a gene of interest can be expressed in only M6 or in both muscles by GAL4-upstream activating sequence expression system. By using this system, we previously demonstrated that expression of activated calcium/calmodulin-dependent protein kinase II (CaMKII) in the muscle cell promotes coordinated maturation of pre- and postsynaptic sites of larvae just after hatching (JAH larvae) in a synapse-specific manner. Here we show that the promotive effects are no longer seen in the older larvae, 8-10 h after hatching (8 h AH larvae). Morphological studies indicate that CaMKII activation in fact reduces postsynaptic sites at 8 h AH. This is opposite to the effect observed in JAH larvae. These results suggest that the mode of CaMKII function switches during development, and that regulation of postsynaptic CaMKII activity is necessary for normal synaptic development. Finally, we report that in 8 h AH but not in JAH larvae, synapses on M7, in which CaMKII activity is not manipulated, are affected by the expression of activated CaMKII in M6. This suggests the interesting possibility that at certain developmental stages only, modification of synapses on one target cell can influence the synapses on another target cell innervated by the same neurons.

Regulation of dendritic growth by calcium and neurotrophin signaling

The development of cortical dendrites is regulated by both activity-dependent and activity-independent signaling. Activity-dependent dendritic growth involves calcium-dependent gene expression. Both CREB and CREST are transactivators that contribute to calcium-dependent dendritic growth. Dendritic development is also regulated by extracellular factors such as neurotrophins. Neurotrophin-dependent dendritic growth is mediated by the MAP kinase and PI 3-kinase pathways. Selective responsiveness to activity cues and neurotrophins may contribute to morphological diversity in the nervous system.

Increased expression of calcium/calmodulin-dependent protein kinase IIβ in frontal cortex in schizophrenia and depression

In searching for genes dysregulated in schizophrenia, we measured the expression of the two splice variants of calcium/calmodulin-dependent protein kinase II (CaMKIIalpha and CaMKIIbeta) in postmortem frontal cerebral cortex tissues from patients who had died with schizophrenia, bipolar disorder, or severe depression. The mRNA levels of expression of these two splice variants were measured by real-time Quantitative PCR, using an Mx4000 instrument. The values for the expression of CaMKIIalpha and CaMKIIbeta were normalized by the expression of beta-glucuronidase in the tissues. The expression of CaMKIIalpha was significantly elevated in the depression tissues by 29%. The expression of CaMKIIbeta was significantly elevated in the schizophrenia tissues by 27%, and in the depression tissues by 36%. Because CaMKIIbeta influences the expression of many neuroreceptors and influences neural outgrowth and pruning, its altered expression in the cerebral cortex in schizophrenia or depression may contribute to these diseases.

Mechanosensory neurite termination and tiling depend on SAX-2 and the SAX-1 kinase

Mechanosensory neurons provide accurate information about stimulus location by restricting their sensory dendrites to nonoverlapping regions, a pattern called tiling. Here, we show that C. elegans sax-1 and sax-2 regulate mechanosensory tiling by controlling the termination point of sensory dendrites. During development, the posterior PLM mechanosensory dendrite overlaps transiently with the anterior ALM mechanosensory neuron. This overlap is eliminated during a discrete period of paused or slowed PLM process growth, between an early period of rapid outgrowth and a later period of maintenance growth. In sax-2 mutants, the PLM sensory dendrite fails to slow between the active growth and maintenance growth phases, leading to sustained overlap of anterior and posterior mechanosensory processes. sax-2 encodes a large conserved protein with HEAT/Armadillo repeats that functions with sax-1, an NDR cell morphology-regulating kinase. High-level expression of sax-2 leads to premature neurite termination, suggesting that SAX-2 can directly inhibit neurite growth.

Cortical rewiring and information storage

Current thinking about long-term memory in the cortex is focused on changes in the strengths of connections between neurons. But ongoing structural plasticity in the adult brain, including synapse formation/elimination and remodelling of axons and dendrites, suggests that memory could also depend on learning-induced changes in the cortical 'wiring diagram'. Given that the cortex is sparsely connected, wiring plasticity could provide a substantial boost in storage capacity, although at a cost of more elaborate biological machinery and slower learning.

Calcium/calmodulin-dependent protein kinase II and synaptic plasticity

A prominent role for calcium/calmodulin-dependent protein kinase II (CaMKII) in regulation of excitatory synaptic transmission was proposed two decades ago when it was identified as a major postsynaptic density protein. Since then, fascinating mechanisms optimized to fine-tune the magnitude and locations of CaMKII activity have been revealed. The importance of CaMKII activity and autophosphorylation to synaptic plasticity in vitro, and to a variety of learning and memory paradigms in vivo has been demonstrated. Recent progress brings us closer to understanding the regulation of dendritic CaMKII activity, localization, and expression, and its role in modulating synaptic transmission and cell morphology.

Mechanisms of dendritic maturation

The highly complex geometry of dendritic trees is crucial for neural signal integration and the proper wiring of neuronal circuits. The morphogenesis of dendritic trees is regulated by innate genetic factors, neuronal activity, and external molecular cues. How each of these factors contributes to dendritic maturation has been addressed in the developing nervous systems of animals ranging from insects to mammals. The results of such investigations have shown that the contribution of intrinsic and extrinsic factors and activity, however, appear to be weighted differentially in different types of neurons, in different brain areas, and especially in different species. Moreover, it appears that dozens of molecules have been found to regulate dendritic maturation, but it is almost certain that each molecule plays only a specific role in this formidable cooperative venture. This article reviews our current knowledge and understanding of the role of various factors in the establishment of the architecture of mature dendritic trees.

Regulation of axonal extension and growth cone motility by calmodulin-dependent protein kinase I

Calcium and calmodulin (CaM) are important signaling molecules that regulate axonal or dendritic extension and branching. The Ca2+-dependent stimulation of neurite elongation has generally been assumed to be mediated by CaM-kinase II (CaMKII), although other members of the CaMK family are highly expressed in developing neurons. We have examined this assumption using a combination of dominant-negative CaMKs (dnCaMKs) and other specific CaMK inhibitors. Here we report that inhibition of cytosolic CaMKI, but not CaMKII or nuclear CaMKIV, dramatically decreases axonal outgrowth and branching in cultured neonatal hippocampal and postnatal cerebellar granule neurons. CaMKI is found throughout the cell cytosol, including the growth cone. Growth cones of neurons expressing dnCaMI or dnCaMKK, the upstream activator of CaMKI, exhibit collapsed morphology with a prominent reduction in lamellipodia. Live-cell imaging confirms that these morphological changes are associated with a dramatic decrease in growth cone motility. Treatment of neurons with 1,8-naphthoylene benzimidazole-3-carboxylic acid (STO-609), an inhibitor of CaMKK, causes a similar change in morphology and reduction in growth cone motility, and this inhibition can be rescued by transfection with an STO-609-insensitive mutant of CaMKK or by transfection with constitutively active CaMKI. These results identify CaMKI as a positive transducer of growth cone motility and axon outgrowth and provide a new physiological role for the CaMKK-CaMKI pathway.

Neural activity and the dynamics of central nervous system development

Recent imaging studies show that the formation of neural connections in the central nervous system is a highly dynamic process. The iterative formation and elimination of synapses and neuronal branches result in the formation of a much larger number of trial connections than is maintained in the mature brain. Neural activity modulates development through biasing this process of formation and elimination, promoting the formation and stabilization of appropriate synaptic connections on the basis of functional activity patterns.

Presynaptic CaMKII is necessary for synaptic plasticity in cultured hippocampal neurons

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a multifunctional enzyme that is very critical for synaptic plasticity and memory formation. Although significant progress has been made in understanding the role of postsynaptic CaMKII in synaptic plasticity, very little is known about its presynaptic function during plasticity changes. Here we report that KN-93, a membrane-permeable CaMKII inhibitor, blocked glutamate-induced increases in the frequency of miniature excitatory postsynaptic currents (mEPSCs) and the number of presynaptic functional boutons in cultured hippocampal pyramidal neurons. In addition, presynaptic injection of the membrane-impermeable CaMKII inhibitor peptide 281-309 blocked synaptic plasticity induced by tetanus, glutamate, or NO/cGMP pathway activation as expressed by long-lasting increases in EPSC amplitude and functional presynaptic boutons. Presynaptic injection of CaMKII itself coupled with weak tetanus produced an immediate and long-lasting enhancement of EPSC amplitude. Thus, the present results conclusively prove that presynaptic CaMKII is essential for synaptic plasticity in cultured hippocampal neurons.

Brain-derived neurotrophic factor induces rapid morphological changes in dendritic spines of olfactory bulb granule cells in cultured slices through the modulation of glutamatergic signaling

While the acute physiological effects of brain-derived neurotrophic factor (BDNF) have been well demonstrated, little is known regarding possible morphological effects that occur within a short period of time. The acute effects of BDNF on dendritic spine morphology were examined in granule cells in cultured main olfactory bulb slices. Organotypic slices prepared from 7-day-old rats were cultured for 1 day, and BDNF was applied at varying time points prior to fixation. Granule cell dendrites were labeled with a membrane dye and observed with a confocal laser scanning microscope. The addition of BDNF into the culture medium 6 h before fixation decreased the mean diameter of the dendritic processes (filopodia/spines), but the length and density of the processes were not affected. Both filopodia/spines in the external plexiform layer and those in the granule cell layer exhibited similar changes. Considering the slow penetration into the slices, BDNF was then applied to the top of each slice. When applied 1 h before fixation, 5 ng and 0.5 ng of BDNF induced the same changes in the external plexiform layer and the granule cell layer, respectively. The changes became detectable as early as 30 min when 50 ng of BDNF was applied. The pretreatment with tetanus toxin or an N-methyl-D-aspartate receptor antagonist abolished the acute effects of BDNF on spine morphology. These results indicate that BDNF can alter spine morphology within a shorter period of time than previously observed and that the effects are mediated by enhanced glutamatergic signaling.

Live Optical Imaging of Nervous System Development

Although development of the nervous system is inherently a process of dynamic change, until recently it has generally been investigated by inference from static images. However, advances in live optical imaging are now allowing direct observation of growth, synapse formation, and even incipient function in the developing nervous system, at length scales from molecules to cortical regions, and over timescales from milliseconds to months. In this review, we provide technical background and present examples of how these new methods, including confocal and two-photon microscopy, GFP-based markers, and functional indicators, are being applied to provide fresh insight into long-standing questions of neural development.

Regulation of the Ca2+/CaM-responsive pool of CaMKII by scaffold-dependent autophosphorylation

CaMKII is critical for structural and functional plasticity. Here we show that Camguk (Cmg), the Drosophila homolog of CASK/Lin-2, associates in an ATP-regulated manner with CaMKII to catalyze formation of a pool of calcium-insensitive CaMKII. In the presence of Ca(2+)/CaM, CaMKII complexed to Cmg can autophosphorylate at T287 and become constitutively active. In the absence of Ca(2+)/CaM, ATP hydrolysis results in phosphorylation of T306 and inactivation of CaMKII. Cmg coexpression suppresses CaMKII activity in transfected cells, and the level of Cmg expression in Drosophila modulates postsynaptic T306 phosphorylation. These results suggest that Cmg, in the presence of Ca(2+)/CaM, can provide a localized source of active kinase. When Ca(2+)/CaM or synaptic activity is low, Cmg promotes inactivating autophosphorylation, producing CaMKII that requires phosphatase to reactivate. This interaction provides a mechanism by which the active postsynaptic pool of CaMKII can be controlled locally to differentiate active and inactive synapses.

N-methyl-d-aspartate receptor activation exerts a dual control on postnatal development of nucleus tractus solitarii neurons in vivo

We have used a morphological approach to evaluate the role of NMDA receptors (NMDAR) in postnatal development of brainstem neurons in awake rats. Chronic NMDAR blockade was performed by placing drug-impregnated Elvax implants over the brainstem at the fifth postnatal day (P5). Compared with control, NMDAR blockade led to a transient increase in dendritic arbor area and filopodium density until P12 followed by a rapid decline in both parameters. Electron microscopy observations showed that these changes correlated with an increase in synapse density at P14 followed by a decrease in synapse density at P28 if chronic NMDAR blockade was maintained until P21. These results support the hypothesis that synapse formation does not require NMDAR activation. In addition, our data suggest a dual role for NMDAR in controlling the synapse number. Early in development NMDARs may be involved in controlling the rate of synapse elimination. Later on, they may subserve synapse stabilization. The physiological significance of these results is discussed.

Postnatal development of dendritic spines on olfactory bulb granule cells in rats

Postnatal morphological changes in granule cell dendritic spines and filopodia (collectively referred to as "spines/filopodia") were examined in the rat main olfactory bulb to characterize the development of the neural circuitry for olfaction. Granule cells were labeled with a membrane dye and confocal laser scanning microscope images of labeled spines/filopodia were acquired in the following three dendritic domains: apical dendrites in the external plexiform layer, those in the granule cell layer, and basal dendrites. In all three domains the proportion of typical spines slightly increased during development, with a concomitant decrease in the proportion of "stubby" spines lacking a neck; the proportion of filopodia remained unchanged, accounting for 20-40% of all protrusions. The mean diameter and length of the spine/filopodium population were nearly constant throughout development. On the other hand, the developmental pattern of the spine/filopodium density varied markedly, depending on the domain of the dendrites. In the external plexiform layer, the density did not change remarkably during development. The density in apical dendrites in the granule cell layer increased during the initial 2 postnatal weeks, then gradually decreased. The spine/filopodium density in basal dendrites, however, continued to increase until 4 weeks of age, and then began to decrease. These results suggest that a substantial amount of input-specific synaptic remodeling occurs in granule cells during development, which proceeds from superficial dendritic domains to deeper ones, occurring most prominently in the basal dendrites.

Organization and evolution of multifunctional Ca2+/CaM-dependent protein kinase genes

The "multi-functional" Ca(2+) and calmodulin-dependent protein kinase, type II (CaMK-II) is an evolutionarily conserved protein. It has been found as a single gene in the horseshoe crab, marine sponge, sea urchin, nematode, and fruit fly, whereas most vertebrates possess four genes (alpha, beta, gamma, and delta). Species from fruit flies to humans encode alternative splice variants which are differentially targeted to phosphorylate diverse downstream targets of Ca(2+) signaling. By comparing known CaMK-II protein and nucleotide sequences, we have now provided evidence for the evolutionary relatedness of CaMK-IIs. Parsimony analyses unambiguously indicate that the four vertebrate CaMK-II genes arose via repeated duplications. Nucleotide phylogenies show consistent but moderate support for the placement of the vertebrate delta CaMK-II as the earliest diverging vertebrate gene. delta CaMK-II is the only gene with both central and C-terminal variable domains and has three to four times more intronic sequence than the other three genes. beta and gamma CaMK-II genes show strong sequence similarity and have comparable exon and intron organization and utilization. alpha CaMK-II is absent from amphibians (Xenopus laevis) and has the most restricted tissue specificity in mammals, whereas beta, gamma, and delta CaMK-IIs are expressed in most tissues. All 38 known mammalian CaMK-II splice variants were compiled with their tissue specificity and exon usage. Some of these variants use alternative 5' and 3' donors within a single exon as well as alternative promoters. These findings serve as an important benchmark for future phylogenetic, developmental, or biochemical studies on this important, conserved, and highly regulated gene family.

The control of dendrite development

Dendrite development is an important and unsolved problem in neuroscience. The nervous system is composed of a vast number of neurons with strikingly different morphology. Neurons are highly polarized cells with distinct subcellular compartments, including one or multiple dendritic processes arising from the cell body, and a single, extended axon. Communications between neurons involve synapses formed between axons of the presynaptic neurons and dendrites of the postsynaptic neurons. Extensive studies over the past decade have identified many molecules underlying axonal outgrowth and pathfinding. In contrast, the control of dendrite development is still much less well understood. However, recent progress has begun to shed light on the molecular mechanisms that orchestrate dendrite growth, arborization, and guidance.

Regulation of cpg15 by signaling pathways that mediate synaptic plasticity

Transcriptional activation is a key link between neuronal activity and long-term synaptic plasticity. Little is known about genes responding to this activation whose products directly effect functional and structural changes at the synapse. cpg15 is an activity-regulated gene encoding a membrane-bound ligand that regulates dendritic and axonal arbor growth and synaptic maturation. We report that cpg15 is an immediate-early gene induced by Ca(2+) influx through NMDA receptors and L-type voltage-sensitive calcium channels. Activity-dependent cpg15 expression requires convergent activation of the CaM kinase and MAP kinase pathways. Although activation of PKA is not required for activity-dependent expression, cpg15 is induced by cAMP in active neurons. CREB binds the cpg15 promoter in vivo and partially regulates its activity-dependent expression. cpg15 is an effector gene that is a target for signal transduction pathways that mediate synaptic plasticity and thus may take part in an activity-regulated transcriptional program that directs long-term changes in synaptic connections.